Generally, a parking brake is mounted on a vehicle in addition to a hydraulic brake such as a service brake, which generates a braking force by hydraulic pressure. The simplest parking brake generates a braking force when an operation lever is pulled, a wire is driven, and a brake pad is activated.
In recent years, an electric parking brake (EPB) is employed, which drives an electric motor to generate or release a braking force when a driver operates an operation switch. In the electric parking brake, when the electric motor is driven, a brake pad is pressed against a disc rotor, so that the braking force is generated.
By the way, when the electric parking brake is activated to park a vehicle, the pad and the disc rotor included in the parking brake are thermally expanded by heat generated by braking while the vehicle is driven. Therefore, if the parking brake is activated while the pad and the disc rotor are thermally expanded, the pad and the disk rotor are cooled and contracted as the time elapses, so that there is a risk that the braking force decreases.
Therefore, in order to maintain a necessary braking force even after time elapses, a method is proposed which compensates the braking force by re-driving the electric motor at regular time intervals after the electric parking brake is activated (for example, see Patent Literature 1).
Also, a method is proposed which estimates the temperature of the pad on the basis of a vehicle state such as a change of vehicle speed and an outside air temperature and determines the time intervals at which the electric motor is re-driven on the basis of the estimated temperature (for example, see Patent Literature 2).